2. M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Brandon E. Bürgler Nonmetallic Inorganic Materials ETH Zürich Single Chamber Solid Oxide Fuel Cells (SC-SOFC) Thursday, March 19 th, 2004

3. M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Outline Single Chamber SOFCs Experiments Modelling Issues Outlook

4. SC-SOFC M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Cathode and anode exposed to same gas mixture of fuel and oxidant Selectivity of electrodes for either oxidation or reduction reaction Single Chamber SOFC OCV O 2 + N 2 H2H2 + N2+ N2 H2OH2O O2O2 ocv CH 4 + O 2 + N 2 CO + CO 2 + N 2 + H 2 CH 4 + N 2 + O2O2 Conventional SOFCSingle Chamber SOFC

5. SC-SOFC M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials SC-SOFC ↔ conventional SOFC Advantages Simplified cell sealing Elimination of complex flow field structures Fast start-up possible Costs Challenges Non-equilibrium gas mixture (explosive from 5 to 15% CH 4 in air) Fuel utilisation? Parasitic chemical reactions

6. SC-SOFC M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Basic Designs of SC-SOFCs CH 4 + air a) classic b) fully porous c) planar

7. SC-SOFC M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Open Questions & Aims Which parameters influence the OCV and the maximum power output? Fundamental model of SC-SOFC including non- ideal electrodes and CH 4 as the fuel High performance SC-SOFC

8. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Measurement Setup CH 4 Air CH 4 Air Exhaust Thermocouple U I Electrical characterisation: Galvanostatic 4-point measurements Temperature: 400 - 700°C dT/dt < 2.5°C/min CH 4 -air mixture: Air: 100-400ml/min CH 4 : 100ml/min, moistened (~3% H 2 O)

9. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Cell Design & Preparation Current collector: Pt-mesh Electrolyte: Ce 0.9 Gd 0.1 O 1.95 (CGO) Anode: 60wt% NiO, 40wt% CGO Cathode: Sm 0.5 Sr 0.5 CoO 3-  10mm 0.2 – 0.53 mm Current collector: Pt-mesh

10. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Fuel Cell Cross Section Cathode (~140  m) Anode (~160  m) Electrolyte (~330  m) Pt-mesh, longitudinal section (~80  m) Pt-mesh, cross section (~80  m )

11. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Open Circuit Voltage MS14 (0.19mm electrolyte):

12. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials U-I Characteristics at different flow T = 600°C

13. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials U-I Characteristics at different flow T = 600°C

14. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials U-I Characteristics at different flow T = 600°C

15. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials U-I Characteristics at different flow T = 600°C

16. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials  P max = 440 mW/cm 2 @ 100 ml/min CH 4 and 200 ml/min Air at 600°C U-I Characteristics at different flow T = 600°C

17. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials U-I characteristic at different Temperatures f Air = 200 ml/min

18. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials U-I characteristic at different Temperatures f Air = 200 ml/min

19. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials U-I characteristic at different Temperatures f Air = 200 ml/min

20. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Open Circuit Voltage

21. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Electrode Temperatures at OCV  Pronounced heat generation on the anode Electrolyte thickness: 390  m

22. Experiments M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Conclusions from Experiments Cells operate at T > 500°C Optimum conditions for maximum Power output at T= 600°C and f air = 300 ml/min Pronounced heat generation at the anode

23. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Modelling of SC-SOFCs 1.Single Chamber SOFC versus Double Chamber: Driving force for ionic current? 2.Calculations of Equilibrium gas mixtures at anode 3.Mixed ionic electronic conducting electrolyte

24. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials What is the driving force for the ionic current? Assumptions: - Hydrogen as fuel, air as oxidant -Reversible and perfectly selective electrodes for H 2 or O 2 -Electrolyte only O 2- -conductor Riess, I., van der Put, P. J. (1995). "Solid oxide fuel cells operating on uniform mixtures of fuel and air." Solid State Ionics 82: 1-4. Cathode ½ O 2 (gas) + 2e - → O 2- (C) Anode ½ H 2 (gas) → H + (A) + 2e - O 2- (SE/A) + 2 H + (A)→ H 2 O (gas) (C) (A) (SE/A)

25. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Calculation of  O 2- Combination of (7), (8) and (9) yields ½ O 2 (gas) + 2e - ↔ O 2- (C) ½ H 2 (gas) ↔ H + (A) + e - O 2- (SE/A) + 2 H + (A)↔ H 2 O (gas) The Nernst Voltage is the same for SC- and conventional SOFCs

26. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Comment Electrodes are not ideally selective nor reversible. Direct oxidation of the fuel (=parasitical) lowers OCV. Improving selectivity of the electrodes will improve efficiency and reduce fuel waste.

27. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Operation Principles of a SC-SOFC O 2 + 4e -  2O 2- p(O 2 ) OCV CH 4 + air H 2 + O 2-  2H 2 O + 2e - CO + O 2-  CO 2 + 2e - CH 4 + 1/2O 2  2H 2 + CO partial oxidation of methane

28. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Modelling of SC-SOFCs 1.Single Chamber SOFC versus Double Chamber. Driving force for ionic current? 2.Calculations of Equilibrium gas mixtures at anode 3.Mixed ionic electronic conducting electrolyte

29. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Calculation of equilibrium gas mixtures Input: T, Composition X(C), X(O), X(H) Minimisation of Gibbs Free Energy Output: Concentrations of species: CH 4, O 2, H 2, CO, CO 2 Thermocalc™ Basic idea: Anode: Equilibrium reached very fast. p O 2 ≈ 10 -26 atm Cathode: non-equilibrium gas mixtures remains unreacted p O 2 ≈ 0.05 – 0.17 atm CH 4 + Air

30. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Equilibria for Different CH 4 :O 2 Ratios Suitable mixtures for SC-SOFCs X (O) T = 600°C

31. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Carbon deposition at low x(O) solid f CH 4 = 100 ml/min f Air = 100 ml/min  Carbon deposition at low x(O)!! gas

32. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Modelling of SC-SOFCs 1.Single Chamber SOFC versus Double Chamber. Driving force for ionic current? 2.Calculations of Equilibrium gas mixtures at anode 3.Mixed ionic electronic conducting electrolyte

33. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials high p O2  conductance predominantly ionic. low p O2  partial reduction  n-type semiconduction Electronic conductivity ~ [Ce 3+ ] ~ p O2 -1/4. D. Schneider, M. Gödickemeier and L.J. Gauckler J. of Electroceramics, 1, [2], (1997), 165-172 Conductivity of Ce 0.8 Sm 0.2 O 1.9-x vs. p O 2  Electrolyte is a mixed ionic electronic conductor -1/4

34. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Transport Model for MIEC- SOFC electrolyte cathode anode Gödickemeier, M., Sasaki, K. and Gauckler, L. J. (1997). "Electrochemical Characteristics of Cathodes in Solid Oxide Fuel Cells based on Ceria Electrolytes." J. Electrochem. Soc. 144(5): 1635-1646.

35. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Partial currents in MIEC – SOFC electrolytes ionic & electronic current I t cell current I t V cell V cell = V OC  -I e = I i open circuit voltage V th electronic current I e V cell electronic current I e I e (V ocv ) V cell = 0  I e = 0 IeReIeRe V cell ionic current I i I i (V ocv ) V cell = V th, app  I i = 0 I i R i +  V th,C +  V th,A

36. Modelling M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Conclusions 1.Single Chamber SOFC versus Double Chamber. Driving force for ionic current? 2.Calculations of Equilibrium gas mixtures at anode 3.Mixed ionic electronic conducting electrolyte 4.Thermal Reactor ----

37. M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Acknowledgements Prof. Dr. L. J. Gauckler A. Nicholas Grundy Michel Prestat SOFC group The entire Nonmets Group Diploma students Marco Siegrist Srdan Vasic

38. M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Thank you for your kind attention